One method of depositing structural layers during manufacture of surface-micromachined devices involves the use of an epitaxial reactor. Epitaxy is a process for producing layers of monocrystalline layers of silicon over a single crystal substrate, and for forming polycrystalline silicon layers over other substrate materials, such as SiO2 films on silicon substrates. Epitaxial reactors may be operated with precisely controlled temperature and atmosphere environmental conditions to ensure uniform deposition and chemical composition of the layer(s) being deposited on the target substrate. In addition to precise control, use of an epitaxial reactor can permit build-up of layers on a substrate at significantly higher rates than typically found with LPCVD systems.
From U.S. Pat. No. 6,318,175, for example, one approach to creating a micromachined device such as a rotation sensor is to apply a sacrificial SiO2 layer to a monocrystalline silicon substrate at a position where one or more micromechanical deflection parts are to be formed. Windows through the SiO2 layer to the silicon substrate may then be formed by applying a layer of photosensitive material, overlaying a mask pattern over the photosensitive layer and exposing the masked surface to light, and then using a developer to selectively remove the light-exposed portion of the photosensitive material and HF to etch the SiO2 layer directly underlying these exposed portions. Following creation of the desired windows in the SiO2 layer, an upper epitaxial layer of silicon may then be deposited on both the SiO2 layer and the contact openings. The upper epitaxial layer grows in polycrystalline form on the SiO2 layer, and in monocrystalline form on the contact window openings to provide a direct connection to the silicon substrate. The structural elements of the micromechanical device may be defined on the upper silicon layer using, for example, an anisotropic plasma etching technique. The etching may be performed through the polycrystalline portion of the epitaxial layer to the SiO2 layer to form trenches around the structural limitations of the micromechanical parts. Finally, the SiO2 layer may be removed from beneath the micromechanical parts in the upper silicon layer during an etching process to complete the formation of the micromechanical device.
The final step of releasing the micromachined structures formed in the upper silicon layer from the underlying sacrificial silicon dioxide layer may be problematic given the following: the geometry of the micromachined structures; the difficulty in ensuring complete etchant penetration through the sacrificial layer beneath the structures; and problems with device deformation and adhesion during the dry process. Release has been accomplished by etching using an HF vapor, as discussed in German Published Patent Application No. 19704454 and U.S. Pat. No. 5,683,591, or by application of liquid HF in combination with supercritical carbon dioxide (CO2), to selectively release and evacuate the sacrificial SiO2 from underneath the micromachined structural parts.
These processes, however, may have associated disadvantages. The chemically aggressive nature of HF may preclude its use in releasing micromachined devices created on substrates cohabited by integrated circuit portions. There may be potential damage due to liquid etchants impinging on delicate micromachined structures. There may be problems created by incomplete elimination of liquid etchants. There may be increased process complexity and expense associated with process steps requiring removal and/or reinsertion of the devices from the epitaxial reactor. There may be a need to maintain the supply and environmental control of materials in special states in an epitaxial reactor environment.
Therefore there is a need for a less-complex, more cost-effective method for releasing micromachined structures from their underlying substrates.